Underground antenna



H. BRUECKMANN 3,435,457

UNDERGROUND ANTENNA March 2 5, 1956-9 Filedvnec. 7. 1965 vllmlll llwlm.. .H

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INVENT f HELMUT aRuEcK fates: Patent U.S. Cl. 343-719 2 Claims ABSTRACT OF THE DISCLOSURE An underground antenna is formed in an excavation with a central upstanding monopole and a top-loading horizontal plate at the top thereof. In order to render the antenna blast-proof, the cavity is hardened with concrete. The hardening material is ordinary concrete with a relative permittivity of two or more with one or more horizontal air gaps. This structure results in a low loss factor and low effective permittivity compared to solid concrete while maintaining the required structural strength. The same principle can be rutilized to provide a low-'loss dielectric for large capacitors, the air gaps being perpendicular to the electric field-lines.

This invention relates to a structural dielectric material exhibiting low loss and also to an underground antenna in which the structural dielectric material is utilized to harden the antenna cavity and thus render it resistant to nuclear blasts and the like.

Underground antennas are often utilized in strategic communications and surveillance systems because they are not easily located by hostile reconnaissance nor are they easily damaged by nuclear or other blasts. It has been proposed to harden certain cavities forming part of an underground antenna system with ordinary concrete or other structural material having relatively high permittivity (er) and a high loss factor (tan While such materials are inexpensive and have the required structural strength, the aforementioned electrical characteristics thereof will cause these antennas to have poor radiation efficiency or undue broadband properties, or both. The present invention comprises an underground antenna in which the structure dielectric material includes at least one air gap, the boundaries of which are essentially equipotential surfaces, that is, the air gap or gaps are perpendicular to the electric field lines. The effective permittivity and loss factor of such a dielectric is considerably reduced compared to solid material as a dielectric and the performance of the antenna is correspondingly enhanced without sacrificing any appreciable structural strength.

It is therefore an object of this invention to provide an improved underground antenna.

Another object of the invention is to reduce the permittivity and loss factor of dielectric used in hardened antennas.

A further object is to provide a novel and useful low loss dielectric material suitable for the hardening of underground antennas or capacitors.

These and other objects and advantages of the present invention will become apparent from the following detailed description and drawing, the sole iigure of which is a cross-sectional View of an antenna embodying the concepts of the present invention.

The illustrated underground antenna comprises a rectangular cavity 4 formed by excavating the earth 3. Centrally located within the cavity is a radiating element comprising a vertical monopole antenna 6 which is an extension of the inner conductor of the coaxial feed line 5. The antenna `6 is centrally connected to a horizontal 3,435,457 Patented Mar. 25, 1969 screen or plate 7 which forms a capacitive top-loading element. The outer conductor of the coaxial feed line 5 is electrically connected to the ground system 11 as it passes therethrough. The ground system may comprise a conductive wire mesh screen or plate. The walls of the cavity are lined with waterproof paper or plastic material 10. lIn accordance with the invention, the cavity 4 is hardened with four layers of concrete dielectric material 8a, Sb, 8c, and 8d, separated by three air Igaps 9a, 9b and 9c. Each layer of concrete rests on the layer below and is spaced therefrom by means of plastic or ceramic spacers 12. Optionally, each spacer may include a pair of metallic cups 13 for distributing the load uniformly over the spacer. The cups 13 may include pins or dowels 14 which are embedded in the concrete. These pins prevent lateral or vertical shifting of the concrete layers caused by seismic or atmospheric waves resulting from a blast. The coaxial feed line 5 is embedded in the lowermost concrete layer 8d. The monopole antenna 6 extends upward through the bore 15 in layers 8b and 8c. The upper part of the monopole 6 and the plate 7 are embedded in the uppermost concrete layer 8a. A water drainage means 16 extends from the lowermost air gap 9c to the earth beneath the cavity 4. A check valve 19 in the drainage line 16 prevents reverse water seepage from the earth to the cavity 4.

It will be apparent that any Voltage existing on the coaxial feed 5 due to either a transmitted or received signal will cause a generally vertical, uniform electric eld to exist between the plate 7 and ground system 11. The electric field lines will therefore be generally perpendiculai` to the air gaps in the concrete, with some small departure from perpendicularity at either side due to fringing effects.

The effective relative permittivity (eeff) and effective loss factor (tan eff) of the concrete dielectric material of the drawing is given by the following relations:

The effective relative permittivity and loss factor are defined as the respective electrical characteristics Of a hypothetical, homogeneous medium which exhibits the same electrical characteristics as the dielectric with the air gap if it is substituted for the latter, the dielectric having an intrinsic relative permittivity of er and a loss factor of tan The factor p in Equations 1 and 2 is the ratio of the width of the air gap or gaps to the total thickness of the material including the air gaps. 'It should be noted that all of these dimensions are measured along or parallel to the electric lines of force. The relative gap p required to halve the loss factor can be found by setting the factor of Equation 2 equal to 1/2 and solving for p. This yields:

1 p- 1 l er For dried concrete with a relative perinittivity of 3.6, a relative gap width p of .22 results in an effective loss factor of one half that which would obtain if solid, homogeneous concrete were used. Thus, it is seen that the reduction in the loss factor is greater than the reduction in the amount of dielectric represented by the air gaps. Further, the greater the intrinsic relative permittivity of the dielectric, the smaller the value of p is required to achieve a given reduction in loss factor. For example,

if e=10, a relative gap Within p of 1/11 Will halve the loss factor. Conversely, for materials with intrinsic relative permittivities which approach unity, for example between 1 and 2, the reduction in the loss factor will be only slightly more, percentagewise, than the reduction in the amount of dielectric material, and the advantages of the layered dielectric Will be only slight. Consequently, the invention is of practical advantage only with structural dielectric materials of intrinsic relative permittivity exceeding 2. For antennas or capacitors having diiferent electric field configuration, relations similar to Equations l and 2 can be Written and eXhibit the same general effect for an air gap, the boundaries of which are substantially equipotential surfaces. Other structural dielectrics having the required values of intrinsic relative permittivity such as granite or asphalt may be used in place of the illustrated concrete.

While the invention has been described in connection with an illustrative embodiment, other applications of the disclosed inventive concepts will be apparent to those skilled in the art. Accordingly, the invention should be limited only by the scope of the appended claims.

What is claimed is:

1. A blast-proof underground antenna comprising a cavity excavated from the earths surface, said cavity ,4. being substantially iilled with a number of layers of concrete dielectric material, said layers being separated by at least one air gap, a radiating element Within said cavity, the electric field produced by said radiating element being substantially perpendicular to said air gap.

2. The antenna of claim 1 in which said radiating element comprises a vertical monopole centrally disposed Within said cavity, said monopole being terminated at its upper end at a horizontal conductive plate, and a metallic ground system beneath the lowermost of said layers of concrete dielectric material.

References `Cited UNITED STATES PATENTS 2,511,610 6/1950 Wheeler 343-910 2,980,793 4/1961 Daniel 343-719 2,989,621 l6/1961 Barton et al 343-719 3,273,152 9/1966 Earp 343-719 3,346,864 10/1967 Harmon 343-719 ELI LIEBERMAN, Primary Examiner.

U.S. Cl. X.R. 343-873, 899 

